A major impediment to the use of ceramic monoliths in certain applications is that when they fail, they do so catastrophicly without any warning of the impending failure. Previous attempts at overcoming the problem of catastrophic failure have entailed reinforcing the ceramic monolith with other materials, i.e. refractory fibers. While the addition of the refractory fibers generally does toughen a ceramic monolith, prevent the catastrophic failure, and allow the ceramic to be used at conventional elevated temperatures, it also destroys some of the desirable properties of the ceramic monolith such as wear resistance and fatigue resistance. Moreover, the incorporation of the fibers creates an entirely new material having substantially different surface and other properties from the base ceramic from which it was prepared. Thus, while the fiber-reinforced materials possess increased toughness and non-catastrophic failure characteristics, they are unsuitable for applications requiring the surface integrity and properties of a monolithic ceramic. And, as new materials, they must undergo extensive evaluation before even being considered for commercial use.
Ceramic monoliths having the same chemical composition have been joined to each other in the past, particularly to produce complex shapes which could not be molded as a single body. The joined bodies, however, have suffered from the same catastrophic failure problems as single monoliths.
Also ceramic monoliths have been joined to metals, particularly for the purpose of putting a wear surface on the metal. Although beneficial for wear performance, these materials still generally exhibit catastrophic failure behavior. Moreover, the vast difference in the coefficients of thermal expansion between metals and ceramics results in high stresses in the structures which (i) can lead to fracture during thermal excursions in use and manufacture and (ii) precludes the use of such structures at temperatures greater than about 400.degree. C. for copper-silver braze connections.
Ceramic light weight armor for helicopter seats and the like has been produced by joining silicon carbide or boron carbide bodies to a polymeric composite of epoxy with Kevlar fibers. The function of the polymeric composite is to stop the shrapnel and hold the ceramic pieces together after fracture from a projectile. The resultant structure is only useful at relatively low temperature due to the presence of the polymeric composite.
Thus if a composition could be developed having (i) the surface and other characteristics of a ceramic monolith, (ii) the non-catastrophic failure characteristics of ductile engineering materials and (iii) the capacity not to deteriorate when exposed to elevated temperatures for extended periods of time, it would find utility in many future applications. Particularly, such a material is needed to produce rolling contact bearings for use in future aircraft engine components. Also it should find use in automobile engine components, aerospace control surfaces, leading edges in aircraft or missile application, turbine engine components, high temperature enclosures, space application including the National Aerospace plane (NASP) engine, as well as in various structural applications.
It is an object of the present invention to produce such a composition which heretofore has not existed.